Generally, semiconductor wafers go through many processing steps before a completed die is formed. Some of these processing steps may include lithography, etching, doping, and depositing different materials. The quality of the completed die may depend largely on the accuracy and precision of the processing between different processing steps. For example, misalignment of a gate structure, imprecise doping concentrations, or dielectric layers that are too thick or thin may cause an undesirable amount of leakage current in a transistor or delay in the operation of the circuit.
Further complicating this problem may be a semiconductor manufacturer's desire to maximize the number of dies produced by the processing steps. In an attempt to maximize productivity, a manufacturer may provide many tools for each processing step. However, each tool may have a behavior different from other tools within the particular processing step. Accordingly, coordinating processing between different tools in different steps may be difficult.
One typical method for coordinating processing includes dispatching lots from tools to specified respective tools in a subsequent stage to compensate for previous processing. This method may consider only the in-line performance of each tool rather than the final quality of the completed product. Further, this method may also suffer from productivity drawbacks because lots from a first tool in a stage are automatically dispatched to a second tool in a subsequent stage without regard for the availability of the second tool.
Another typical method attempts to control tool behavior and does not have any set dispatching rules. One example is an Automatic Process Control (APC) that uses in-line measurements after processing a lot to control the tool that previously processed the lot and to control the tool that will immediately subsequently process the lot. This method may consider only productivity, such as minimizing tool idle time, and may ignore the mismatch impact between tools. Thus, the quality of a finished die may be adversely impacted.